Posted by **vett** on Tuesday, May 31, 2011 at 9:59pm.

in abc, c=60 degrees,a=12, and b=5. find c.

- math -
**MathMate**, Tuesday, May 31, 2011 at 10:15pm
We usually use Uppercase letters to represent angles, and lowercase for side lengths.

I resume the question as follows:

in abc, =60°,a=12, and b=5. find c.

We can solve this using the cosine rule:

c²=a²+b²-2abcos(C)

Substitute the numbers and take the square-root of c² to get c.

- math -
**Anonymous**, Tuesday, May 31, 2011 at 10:20pm
Use law of cosines:

c^2 = a^2 + b^2 - 2*a*b*cos(C)

c^2 = 12^2 + 5^2 - 2 * 12 * 5 * cos(60°)

c^2 = 144 + 25 - 120*cos(60°)

c^2 = 169 - 120 * 0.5

c^2 = 169 - 60 = 109

c = sqrt(109) = 10.4403

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